Pitot tube type flow measurement devices insertable into a fluid flowing within a pipe comprise commonplace instruments for measuring flow. Differential pressure measurements constitute the data from which the flow rate can be calculated and, since these measurements are taken internally but read out externally, it is necessary that the probe be provided with an external fluid-tight mounting of some type attached to the pipe. Such mountings are also well known in the art and may, for example, include shut-off valves such that the fluid entering the mounting can be contained while the probe is removed altogether. Other situations exist where the probe must be retracted to a position outside the pipe so as to not obstruct the latter yet remain attached thereto by means of the mounting. One such situation is that in which the pipe is to be cleaned by running a so-called "pig" through it. Obviously, the presence of a probe in its extended operative position would make such an operation impossible.
Applicants and others have, over the years, developed a number of different systems for inserting and withdrawing probes of the type mentioned above. One of the most common systems for accomplishing this task is to attach the probe to a piston and use compressed air or a fluid to reciprocate the piston pneumatically or hydraulically. While such systems have the capability of being designed for actuation from a remote location, to do so becomes expensive and is hardly worth the price when the occasion for its use arises so infrequently. Moreover, and most important, is the fact that such systems are unreliable and if they fail for some reason which they often do, then not only is the probe damaged but the pipeline is plugged. Accordingly, applicants have found that, because the only safe course to follow is to monitor the probe-withdrawal operation on site anyway, all the expensive sophistication is surplusage for the most part and it is, therefore, considered far better to provide a simple manually-operated insert/retract mechanism which can be relied upon to withdraw the probe as well as provide a visual indication that it has, in fact, been removed. The design of such a mechanism is not, however, without its problems.
Among these problems is the fact that extreme care must be exercised so as to not damage the probe or the fluid-tight seal that permits it to be reciprocated without leaking fluid or causing a loss in line pressure. Also, considerable attention has to be given to the fact that one must limit the excursion of the probe in both directions or, otherwise, it can be crushed by striking the remote pipe wall as it is inserted or pulled out past the packing that seals it within its mounting. Last, but by no means least, is the necessity for insuring that the insert/retract mechanism employed is capable of resisting the sometimes considerable line pressure acting to expel it.